Alpha Male,
JAR ops requirements for single pilot air taxi is 700 total time with at least 40 p1 multi IFR. Due to the shortage some operators are now taking people with the minimum, however it was quite common when the job market wasn't quite so good for air taxi firms to ask for 1000 hours total with between 100 to 200 multi.
Centreline air charter have a concession such that they are able to employ pilots with 400 hours total time for single pilots ops, but I believe this is the only the company to my knowledge with this concession ( there may be one or two others of course). I Infact the post where you read the 400 tt requirement was requirement was written by the manager of Centreline.
f you are lucky as a safety pilot for an air taxi firm you may be able to log the multi time flying empty sectors, but otherwise to obtain the multi-time you may be looking at spending around 6000 pounds on an hour building trip to the states (100 pounds an hour is about the cheapest hire for twins in the US, so take in account all your other expenses). However the people I have known to get safety pilot postion in order to obtain the mulit time for single pilot ops have tended to be instructors with 700-1000 hours. At the moment any one with 700-1000 hours has a good chance of getting an interview with a jet or turboprop operator (see my previous post).
So if you spend 6000 on the twin time at low hours Centreline is the only air taxi firm that could employ you; just because you have the multi time doesn't mean you will pass the company base check.